Amity - Corrosion - Why So Dumb

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Contents Introduction

2

Dry Corrosion

2

Oxidation Corrosion

3

Liquid Metal Corrosion

4

Corrosion by Other Gases

5

Wet Corrosion

5

Rusting of Iron

5

Types of Corrosion

7

Galvanic corrosion

7

Differential Aeration Corrosion

8

Stress Corrosion

9

Factors Affecting Corrosion

10

Nature of Metal

10

Nature of Environment

11

Preventive Measures of Corrosion

13

Microbiological Corrosion

15

Introduction Corrosion is the process of slowly deterioration and consequent loss of a solid metallic material from the metallic surfaces due to attack of atmospheric gases, soil, chemical or electrochemical environment. It is a very slow process and starts from the surface of the metal.

Corrosion is basically an oxidation process in which metallic compounds having lower energy is formed and energy liberates.

corrosion Metal (Higher energy)

Metallic Compound + Energy (Lower energy)

Dry Corrosion It is also known as chemical corrosion. Such types of corrosion take place due to direct chemical action of atmospheric gases like CO2, SO2, O2 and H2.

It is of three types: i) Oxidation Corrosion ii) Liquid Metal Corrosion iii) Corrosion by other gases

Oxidation Corrosion When corrosion takes place by direct action of oxygen on metal. It occurs usually in the absence of moisture. Oxidation corrosion occurs at ordinary temperature. Alkali metals and alkaline earth metals undergo oxidation corrosion.

Mechanism: When a metal is exposed to air it gets oxidized by losing its valence electrons and reduction of oxygen takes place.

M

Mn+ + ne¯

n/2 [½O2 + 2e¯ M + n/4O₂

O²-] Mn+ + O²-

Oxidation of metal Reduction of oxygen

At the point of contact of Mn+ and O²- , metallic oxide M2On is formed. This metallic oxide scale forms a barrier to restrict further oxidation of metals. The nature of oxide film plays very important role in oxidation corrosion.

i) When the oxide film is stable and tight, it will act as a protective coating and corrosion is further prevented. For example, Al, Pb and Cu.

ii) When the oxide film is unstable and tendency to decompose, it does not undergo oxidation corrosion.

iii) When the oxide film is volatile and sufficiently porous, oxidation corrosion takes place continuously.

Liquid Metal Corrosion When a liquid metal is allowed to flow over solid metal at high temperature, it is called liquid metal corrosion. As a result, solid metal gets weakened.

Corrosion by Other Gases Gases like SO2, Cl2 and H2S react with certain metals and form a protective or non-protective layer on metallic surface. Due to the chemical combination of metals with gases, metals undergo corrosion and such type of corrosion is produced by gases.

Wet Corrosion It is also known as electrochemical corrosion. Such type of corrosion is due to the flow of electrons from anode to cathode in the presence of conducting solution. It is a very common type of corrosion.

Rusting of Iron When an iron piece is exposed to moist air or mixture, a reddish brown layer is formed on the iron surface. This reddish brown layer is called the rust and the phenomenon as rusting. The main component of rusting is ferric oxide (Fe2O3.xH2O) or iron oxide (Fe3O4).

Note: Both O2 and H2O are essential for rusting because Fe will not rust in dry air or in water that is completely free from oxygen.

Factors involved in rusting: i) Presence of air ii) Presence of moisture iii) Presence of electrolytic impurities in water iv) Presence of impurity in metals v) Nature of metals

Mechanism of rusting of iron:

The mechanism of rusting is based on electrochemical theory. H2O + C2O

H2CO3

Due to acidic nature, ionization of acid takes place H2CO3 H2O

Oxidation at cathode:

2H+ + CO32H+ + OH¯

Fe H+ + e¯ 4H+ + O2 + 4e¯

Fe²+ + 2e¯ H 2H2O

Reduction at cathode: [ H+ + e¯

H]

4H+ + O2

2H2O

O2 + 4H+ + 4e¯

2H2O

x4

Types of Corrosion i) Galvanic corrosion: When different metals are in contact either directly or through an electrical conductor and are exposed to a corrosive atmosphere, the metal which has more negative electrode potential undergoes corrosion. Such type of corrosion is called galvanic corrosion. Differential galvanic corrosion is called galvanic corrosion. Mechanism:

The galvanic corrosion occurs due to the difference in the electrode potential of metals. The metal which has lower standard reduction potential act as anode and under goes corrosion. The electrons flow from the anode to the cathode and galvanic cell is setup. As a result the anode undergoes oxidation and forms corresponding ions and passes into the solution. The process continues and metal goes corroded gradually.

Galvanic corrosion can be controlled by:

(a) Avoiding the suitable medium for corrosion. (b) Minimizing the potential differences of metal i.e. avoiding the galvanic couple. (c) By polishing the metal.

ii) Differential Aeration Corrosion: It also known as concentration cell corrosion and occurs due to electromagnetic attack on the metal surface when a metal is exposed to an electrolyte of different concentration. The corrosion occurs when a portion of metal is exposed to a different air concentration.

Important Characteristics of Different Aeration Corrosion:

(a) Metal having low oxygen concentration part act as anode and metal having oxygen concentration part act as anode. (b) It is promoted by accumulation dirt sand scale or other contamination, which act as anode and involves in oxidation. (c) It is localized attack on some oxygen deficient areas and result in pitting.

iii) Stress Corrosion: This type of corrosion occurs due to combined effect tensile strength and the corrosion environment on metal where it is exposed to corrosion environment. Pure metal does not undergo stress corrosion whereas fabricated metal component undergoes this process.

Condition for Stress Corrosion:

(a) Tensile stress, which is caused by quenching, bending, annealing, welding etc. (b) Corrosion environment.

Types of stress corrosion:

(a) Season cracking: It is generally applied to the stress corrosion of copes alloy mainly brass. It occurs in presence of ammonia or amine. (b) Caustic embattlement: It is covered by usually highly alkaline water in high pressure boils.

Factors Affecting Corrosion Corrosion of metal prominently depends on: i) Nature of metal ii) Nature of environment Nature of Metal

(a) Impurities: Present in metal or alloys, these enhance the rate of corrosion by forming minute electrochemical cells under suitable environment conditions and undergo corrosion.

(b) Series: The metal or alloys which are placed at higher position in the series are more reactive, thus undergoes corrosion.

(c) Physical State: The rate of corrosion is also influenced by physical state (size of crystal, orientation of crystal). Smaller the gain size of metal, greater is the corrosion.

(d) Nature of Oxide film: All metals get covered by a very thin film of oxide. Greater the specific volume ratio of oxide film, lesser is the rate of corrosion.

(e) Volatility: When the corrosion product is volatile, it volatilizes as soon as it is formed, thus exposing the underlying metal surface for further attack. Hence in such a case, corrosion goes on increasing with time.

Nature of Environment

(a) Effect of temperature: Increase in temperature increases the rate of corrosion.

(b) Effect of pH: Corrosion is more in acidic media (pH < 7) than in alkaline media (pH > 7) and neutral media (pH = 7).

Amphoteric metals like Al, Zn and Pb form complexes in alkaline media. Hence, such types of complexes undergo corrosion in alkaline media.

(c) Effect of corroding medium: The conductive corroding medium increases the rate of corrosion.

(d) Effect of concentration of oxygen: The rate of corrosion increases in the presence of moisture and oxygen.

(e) Atmospheric moisture: It acts as a very good solvent for oxygen and form electrochemical cell. This rapidly increases the rate of corrosion. (f) Effect of suspended particles in atmospheres: Suspended active particles like NaCl, (NH4)2SO4 etc present in the atmosphere absorb moisture rapidly due to hydroscopic nature and act as strong electrolytes which increases the rate of corrosion. (g) Effect of nature of the presence of electrolytes: The presence of electrolyte in the corroding environment play an important role in corrosion. Due to the presence of electrolytes, corrosion takes place rapidly.

Preventive Measures of Corrosion Corrosion can be controlled by following ways:

(i) Protective coatings: It is very common and simplest method for prevention of metals from corrosion.

It can be done by: (a) Surface Coatings: Corrosion can be controlled by using paints, enamels, layers, oil, grease or non corroding metals (Ni, Cr) as an electroplating material. (b) Using antirust solutions: Alkaline phosphates and alkaline chromates are used as antirust solution to prevent corrosion. (c) Anodic Coatings: Coating of anodes like Zn or Al on steel surface prevents the steel from corrosion.

ii) Electroplating: It is the most widely used method of coating metals. It is the method for prevention of metals from corrosion. The process is carried out in an electrolytic cell. This method has a great importance in industries for the prevention of metals part in machines for corrosion.

iii) Galvanization and Tinning: It is the method of coating by metals and alloys having low melting point like Zn, Sn, Pb, etc on the surface of metals to be coated. Coating of zinc on iron is called galvanization and tin on iron surface is called tinning.

iv) Proper Design: The metallic apparatus must be properly designed to minimize the corrosion.

Following points should be considered for designing:

(a) Avoid the contact of dissimilar metals in the presence of a corroding solution. (b) When two dissimilar metals are to be in contact, the anodic material should be in a large area as much as possible to minimize the corrosion. (c) An insulating fitting should be applied for joining two dissimilar metals. Jointly of direct metal-metal electrical contact should be avoided.

Microbiological Corrosion Microbial Corrosion is the type of corrosion which takes place microbiologically. It is influenced by presence and activities of microorganism and their metabolites. Metabolites are the products produced in their metabolism. Bacteria, fungi and other microorganisms can play a major part in soil corrosion.

Sulphate reducing bacteria plays very important role in the corrosion. Sulphur bacteria (thioracillus) are responsible for the aerobic corrosion. They oxidize the sulphur present in the cell organelle sulphuric acid which is sufficient for the corrosion of iron. Their activities are optimum in acid medium pH 7. Iron and manganese are aerobic and digest the iron and manganese ions in cells to form insoluble hydrates of iron and manganese dioxide. These are excreted by cells. Iron bacteria grows in stagnant or running in water below 40oC and pH 4 - 10 with a small amount of free dissolved oxygen.

Mechanism of anaerobic microbiological corrosion of iron: 8H2O

8H+ + 8OH-

4Fe + 8H+

4Fe2+ + 8H

Depolarization due to bacterial activity: H2SO4 + 8H

H2S + 4H2O

Corrosion products: Fe2+ + H2S

FeS + 2H⁺

3Fe2+ + 6OH-

3Fe(OH) 2

This type of corrosion is intense and localized.